It is estimated that 10% of the United States population will suffer from urinary stone disease during their lifetimes. The National Institute of Diabetes and Digestive and Kidney Diseases reports that more than 1 million cases occur annually. Patients often experience acute onset of intense pain as the first symptom of the presence of a hardened mass in the urinary tract, which typically lands them in the emergency room of a hospital. Evaluation and treatment of urinary stone disease costs billions of dollars annually.
Patients with small stones normally are advised to wait to pass the stone. For more severe urinary stone disease, physicians commonly perform a lithotripsy procedure in which the physician breaks apart stones that are too large to pass successfully. One typically employed method for removing large urinary tract stones is endoscopic lithotripsy, generally performed with an ureteroscope, which is a specialized fiber optic endoscope designed to navigate the urinary tract. The endoscope may have as one of its components an optical fiber of a length and flexibility to stretch from a laser at the proximal end of the fiber to the stone's location at the distal end of the fiber inserted into the urethra, bladder, or kidney. Once the treatment tip of the fiber is in close proximity or even in contact with the stone, energy is transmitted through the fiber to the stone.
Optical fibers normally comprise a core host material, a cladding, and a jacket. Typically, the treatment fiber that is connected to the laser system and inserted into the body comprises low-OH silica multimode fiber that includes a solid core of silica, a silica cladding of a different index of refraction to direct the light back into the core for propagation, and a polyimide buffer or other suitable plastic jacket. The core host is doped with an impurity that changes the refractive index of the core, trapping light in the core so that the light is transmitted axially. The silica core typically is doped with germanium. The cladding circumscribing the surface of the core directs light energy back into the core to propagate the light along the length of the fiber, which is commonly referred to as “total internal reflection.” The jacket circumscribing the cladding protects the cladding and the core within, but does not aid in light transmission.
For endoscopic laser lithotripsy, many physicians use the Holmium:YAG (Ho:YAG) laser, which is a flashlamp pumped solid-state laser having within the laser housing a solid crystal lasing medium comprising a host of yttrium, aluminum, and garnet doped with holmium. The pump source stimulates the spontaneous and amplified emission of photons in the solid crystal lasing medium, providing energy to the laser system. The Ho:YAG laser provides a multipurpose system having a relatively high power output that can be used to cut or coagulate a variety of soft and hard tissues.
However, because flashlamps produce a broad spectrum of light, much of the energy is wasted as heat in the gain medium. Ho:YAG laser systems produce a laser beam that includes a relatively large and multimode beam waist, departing from the ideal single-mode Gaussian beam, and has only limited flexibility for pulse rate and duration. Problems may occur, including thermal lensing, potentially leading to misalignment of the laser beam with the proximal end of the treatment optical fiber, which is the treatment fiber input end attached to the laser source. Suboptimal coupling from the large multimode beam profile can damage the treatment fiber on the proximal end and the treatment fiber must then be replaced.
The Ho:YAG laser also tends to subject the distal, or treatment end, of the treatment fiber to damage, at least in part because the fibers are not sufficiently flexible to avoid damage from bending in the tight spaces of the urinary tract, particularly the lower pole of the kidney. Semi-rigid ureterscopes typically use relatively large and somewhat less flexible 365 μm fibers. Flexible ureteroscopes normally have a single working channel of 1.2 mm inner diameter that provides limited valuable space for saline irrigation and thus typically are used with smaller fibers having a core no larger than 270 μm. Larger fibers can be accommodated, but reduce flexibility and limit saline irrigation. However, the Ho:YAG laser's large beam waist prevents optimal power coupling into fibers smaller than 270 μm. The short bending radius at the distal end of a 270 μm fiber decreases the longevity of these fibers. Over multiple cycles of bending the laser energy leaks from the core into the cladding to damage the fiber. Stone fragmentation can also damage the fiber on the distal end and the entire treatment fiber must then be replaced, even if the proximal end is yet undamaged.
Smaller fibers of about 200 μm or less could provide more flexibility, especially for accessing hard-to-reach locations in the lower pole of the kidney, and could provide more room in the working channel of the ureteroscope for irrigation with saline, which would improve visibility for the physician and safety for the patient. Nevertheless, optical coupling into these smaller fibers with the Ho:YAG laser's multimode beam profile risks overfilling of the input fiber core, launching laser energy into the cladding, and damaging the treatment fiber.
Proximal tapered fiber tips, in which the fiber has a steep taper at the proximal end from a larger diameter to a smaller trunk diameter, have been used to more efficiently couple Ho:YAG laser energy into fibers with smaller trunk diameters. However, higher order modes are created once the small core size is reached at the distal end of the taper. These modes fall outside the total internal reflection condition of the fiber. Laser energy typically escapes into the fiber cladding, potentially degrading fiber integrity. With extremely tight bending of the fiber, such as with ureteroscope deflection into the lower pole of the kidney, these leaking modes may increase and laser energy may escape. Escaping laser energy can burn through the ureteroscope wall, damaging the ureteroscope and potentially harming the patient.
Single use and reusable treatment fibers are available for Ho:YAG laser lithotripsy, with 270 μm fibers common in flexible ureteroscopes and the even larger 365 μm fibers common in semi-rigid ureteroscopes. Procedures for reuse include stripping the fiber jacket, cleaving the tip, and sterilizing the fiber after each procedure. Reprocessing for reuse costs little compared to replacement of single use fibers. Reusing Ho:YAG laser lithotripsy fibers by cleaving the damaged tip has been reported to decrease single procedure costs by an average of $100 US. Reusable fibers are not typically discarded until they become too short after multiple cleavings or they become irreparably damaged from bending or suboptimal coupling.
The thulium fiber laser (TFL), which uses a thulium doped silica fiber as the lasing medium instead of the holmium doped crystal of yttrium, aluminum, and garnet, has been proposed as an alternative laser lithotripter to the clinical Ho:YAG laser for several reasons. First among them is that the TFL has 4 times higher absorption in tissues at equivalent pulse energies compared to the Ho:YAG laser, which means that the TFL can ablate stones at 4 times lower pulse energies than the Ho:YAG laser. The TFL is pumped with a laser diode energy source instead of a flashlamp, which provides for a more efficient energy transfer to the optical fiber. Generally speaking, flashlamp pumped lasers, including the Ho:YAG laser, are limited to fixed pulse lengths at lower pulse rates than the TFL. The TFL is more compact than the Ho:YAG laser and can be electronically triggered to operate at nearly any pulse length or configuration. The TFL can be modulated to create pulse trains. The micropulse train, also called a “pulse packet” mode of operation, enables increased laser ablation rates for both soft tissues and hard tissues, including urinary stones, for the TFL as compared to the Ho:YAG laser.
The TFL has a higher absorption coefficient and shorter optical penetration depth in water. The TFL absorption coefficient is 160 cm−1 compared to 28 cm−1 for the Ho:YAG laser. The most common urinary stones are hydrates, and the stones typically are immersed in a fluid environment, including saline irrigation that is used to improve visualization during lithotripsy. Water absorption is an important factor even though urinary stones typically have significantly less bound water content than soft tissues. Water absorption of laser energy is correlated with the ablation threshold and is dependent on the wavelength of the laser. The TFL has two major emission wavelengths at 1908 and 1940 nm, which closely match both a high and low temperature water absorption peak, respectively. The TFL absorption peak shifts from 1940 nm at room temperature to 1910 nm at the higher temperatures encountered when water associated with urinary stones is superheated during laser tissue ablation, including both water bound in the stone and unbound water surrounding the stone.
Another advantage of the TFL is its Gaussian spatial bean profile compared to the Ho:YAG laser's multimode beam. The superior spatial beam profile of the TFL improves coupling and transmission of laser power through smaller diameter treatment fibers for lithotripsy, allowing use of fibers of core diameters of from about 200 μm or less. This reduction in fiber cross-section would be expected to allow for increased ureteroscope deflection and higher saline irrigation rates through the working channel, which, in turn, could reduce procedure times, probability of ureteroscope damage, and physician visibility, improving patient safety. However, the smaller diameter distal fiber tips on the treatment fiber have been shown to degrade and suffer from “burn-back” more than larger diameter fiber tips, which means the entire fiber has to be replaced more frequently during the procedure.
The TFL's Gaussian beam profile, which is an indication of the spatial intensity, width and quality of the laser beam, provides improved coupling into small treatment fibers, eliminating proximal fiber tip damage as compared to the Ho:YAG. By reversing the typical orientation of the tapered fiber, and using the increasing taper and larger core at the distal output end instead of the laser proximal input end, the distal tip of the continuous treatment fiber has been shown to be more damage resistant during lithotripsy. The benefits of a small-core trunk fiber, including increased irrigation and flexibility, are then combined with that of a robust larger-core distal fiber tip. The tip can be extended from the ureteroscope into contact with the stone, while also providing sufficient irrigation since only the small-core trunk fiber remains within the working channel of the ureteroscope. Other potential advantages of a small trunk fiber having a tapered distal fiber tip include less divergence of the output beam and a larger treatment area.
Nevertheless, fiber tip damage and burn-back still occur with large tips and the entire fiber must be replaced or repaired during or after the procedure, so impediments remain to adoption of the TFL for lithotripsy.
It would be desirable to provide an ureteroscope in combination with a fiber delivery system that has the advantages of a TFL of compact size, increased ablation rates at lower energies, improved irrigation, reduced damage at the proximal end in alignment with the laser source, and reduced fiber tip damage at the distal end of the treatment fiber, and yet eliminates or otherwise improves upon the disadvantages currently experienced, including the problems of burn-back, fiber replacement during or after each procedure, and fiber reconditioning for reuse.